A 7.0-magnitude earthquake has struck the capital of Haiti, raising fears of hundreds of deaths in the Caribbean's most troubled country.
Switzerland has sent a quick-response team to Port-au-Prince to assess needs.
Haiti's worst quake in two centuries struck late in the afternoon local time about 16 kilometres southwest of Port-au-Prince, and was quickly followed by two strong aftershocks of 5.9 and 5.5 magnitude.
Serious damage has been reported to the headquarters of the United Nations mission in Haiti where a five-storey building collapsed and a "large number" of personnel are unaccounted for. Between 200 and 250 people normally work in the building.
The national palace, a hospital and a big hotel are among other buildings badly damaged, and rescuers are digging under rubble.
According to the French government, only 100 of the 300 people staying at the hotel were able to get out of the building.
Haiti's envoy to the United States said it was a "catastrophe of major proportions".
Rescue and aid efforts
The International Red Cross and Red Crescent Movement, based in Geneva, said disaster response teams would arrive in Haiti later on Wednesday.
It said they would coordinate international assistance from Red Cross members.
The most urgent needs, according to the Red Cross, are search and rescue, field hospitals, emergency health, water purification, emergency shelter, logistics and telecommunications.
A spokesman for the International Committee of the Red Cross said the Haitian Red Cross was completely overwhelmed and that there was very little information about the scale of the disaster.
The Swiss foreign ministry has offered Switzerland's assistance to the Haitian government.
A Swiss quick-response team left for Haiti late on Wednesday morning. Its task, according to the foreign ministry is to "strengthen" the Swiss embassy and the ministry's development aid office "to identify what needs to be done and to initiate emergency measures".
Around 130 Swiss nationals are believed to reside in Haiti. At the moment, the ministry said, none are among the victims.
"The whole city is in darkness. You have thousands of people sitting in the streets with nowhere to go," said Rachmani Domersant, an operations manager with the Food for the Poor charity. "There are people running, crying, screaming."
"I think hundreds of casualties would be a serious understatement."
Witnesses said they saw homes and shanties built on hillsides come tumbling down as the earth shook.
UN officials said normal communications had been cut off and the only way to talk with people on the ground was via satellite phone. Roads were blocked by rubble.
The quake prompted a tsunami watch for parts of the Caribbean but this was later lifted.
swissinfo.ch and agencies
Swiss Solidarity, a charity led by swissinfo.ch's parent body, the Swiss Broadcasting Corporation, is asking for donations for the victims of the earthquake.
The charity said it would work alongside Swiss relief agencies in the coming months to aid victims and support reconstruction efforts.
The charity is already financially engaged in several development aid programmes in Haiti.
Donations can be made through a special postal account: 10-15000-6.
Haiti is the poorest country in the Western Hemisphere and has a history of natural disasters.
Along with the Dominican Republic, it occupies the island of Hispaniola.
Some 9,000 UN police and troops are stationed there to maintain order.
The last earthquake of this magnitude was in 1770.